Sensor system for buried waste containment sites

ABSTRACT

A sensor system is disclosed for a buried waste containment site having a bottom wall barrier and/or sidewall barriers, for containing hazardous waste. The sensor system includes one or more sensor devices disposed in one or more of the barriers for detecting a physical parameter either of the barrier itself or of the physical condition of the surrounding soils and buried waste, and for producing a signal representing the physical parameter detected. Also included is a signal processor for receiving signals produced by the sensor device and for developing information identifying the physical parameter detected, either for sounding an alarm, displaying a graphic representation of a physical parameter detected on a viewing screen and/or a hard copy printout. The sensor devices may be deployed in or adjacent the barriers at the same time the barriers are deployed and may be adapted to detect strain or cracking in the barriers, leakage of radiation through the barriers, the presence and leaking through the barriers of volatile organic compounds, or similar physical conditions.

RELATED APPLICATION

[0001] This application is a Divisional of pending and allowed U.S.Application Serial No. 09/418,681, filed on Oct., 14, 1999.

CONTRACTUAL ORIGIN OF THE INVENTION

[0002] The United States has rights in this invention pursuant toContract No. DE-AC07-94ID13223 between the U.S. Department of Energy andLockheed Martin Idaho Technologies, Inc.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates generally to a sensor system formonitoring the structural integrity of an underground waste containmentbarrier, and leakage therefrom of waste products or byproducts, and forimproved characterization of zones of interest.

[0005] 2. Background Art

[0006] It is often necessary to form a containment barrier around ahazardous waste site to stop or prevent the migration of contaminantsinto the nearby soil and water tables. The containment barrier mustprevent the migration of contaminants both horizontally and verticallyaway from the waste site. Therefore, a properly constructed containmentbarrier may be compared to a huge bathtub, with the hazardous wastecontained within four side walls and a generally horizontal floor.

[0007] A typical, currently-used method of containment is to physicallyremove the hazardous waste and haul it to a permitted storage facility.However, such method is costly, impractical, and dangerous. Digging upsites with buried drums, radioactive dusts, or other airborne wastes mayactually release the contaminants, spreading them into the atmosphereand through the soil.

[0008] In response to this problem, a number of suggestions have beenmade for placing containment barriers around hazardous waste sites,without removing the waste. One approach for doing this is disclosed inInternational Publication Nos. W0 94/19547 and W0 93/00483 byHalliburton Nus Environmental Corp. The Halliburton system uses a row ofhigh pressure jets to shoot a slurry into soil surrounding a hazardouswaste site, somewhat liquefying the surrounding soil. The slurry cuts apath through the soil as it intermixes with the liquified soil. Gravityand/or mechanical means pull the row of high pressure jets through themix of liquified soil and slurry, after which the liquified soil andslurry harden into a protective barrier.

[0009] The above-described system has a number of shortcomings,including the possibility of further spreading contaminants by the useof hydraulic jets, the difficulty of maintaining balance between theamount of slurry needed for cutting and the amount of slurry needed forhardening the soil, the difficulty of providing a barrier of consistentstrength since it would depend in part upon the soil compositionencountered and the amount of slurry deposited, and, finally, the lackof testing of excavated soil to know whether soil surrounding the wastesite has become contaminated.

[0010] Another suggested approach for installing a containment barrieraround a hazardous waste site is disclosed in co-pending patentapplication Ser. No. 08/______ , filed ______. In this approach, amulti-layer containment barrier is put in place under a hazardous wastesite without disturbing any buried waste, in a simple and efficientfashion. The disclosure in the above-noted co-pending patent applicationis incorporated herein by reference.

[0011] In any approach to holding hazardous waste, it would be desirableto monitor the site in terms of both the structural integrity of anycontainment barrier put in place about the waste material, and leakageof contaminants away from the site. Additionally, it would be desirableto monitor material being excavated from around a waste site inpreparation for emplacement of a containment barrier for the site, todetermine the extent of contamination of surrounding soils and thus thepossible need to extend the containment barrier to a location completelysurrounding all contaminated materials and soils. Finally, it would bedesirable to efficiently and inexpensively install a long-termmonitoring system soon after or simultaneously with the installation ofthe containment barrier.

OBJECTS AND SUMMARY OF THE INVENTION

[0012] It is an object of the invention to provide a sensor system forsensing a variety of physical parameters of a buried waste containmentsite.

[0013] It is also an object of the invention to provide such a sensorsystem especially suitable for use in connection with a containmentbarrier disposed under and around a buried waste site.

[0014] It is a further object of the invention to provide such a sensorsystem for monitoring the structural integrity of such a containmentbarrier.

[0015] It is also an object of the invention to provide such a sensorsystem for sensing leakage of contaminants from a buried wastecontainment site.

[0016] It is still another object of the invention to provide such asensor system, in accordance with one aspect thereof, for monitoringsoil and material excavated from a buried waste containment site.

[0017] It is an additional object of the invention to provide such asensor system, in accordance with another aspect thereof, for sensingphysical parameters of soil being excavated, during the excavationprocess.

[0018] It is a further object of the invention to provide such a sensorsystem which may be readily installed at a buried waste containment sitesimultaneously with the installation of a containment barrier.

[0019] It is also an object of the invention to provide such a sensorsystem in which sensors may be installed and removed after the buriedwaste containment site is in place.

[0020] The above and other objects of the invention are realized in aspecific illustrative embodiment of a sensor system for a buried wastecontainment site having a bottom wall barrier and/or sidewall barriers,for containing hazardous waste. The sensor system includes one or moresensor devices disposed in one or more of the barriers for detecting aphysical parameter either of the barrier itself or of the physicalcondition of the surrounding soils and buried waste, and for producing asignal representing the physical parameter detected. Also included is asignal processing device for receiving signals produced by the sensordevice and for developing information identifying the physical parameterdetected, either for sounding an alarm, displaying a graphicrepresentation of the physical parameter detected on a viewing screenand/or a hard copy printout, etc.

[0021] In accordance with one aspect of the invention, the sensor devicedisposed in one or more of the barriers comprises a strain or cracktransducer for detecting strain or cracking and thus possible leakagelocations in the barrier in which the transducer is disposed. Oneembodiment of such a transducer includes a grid of detecting elementsdisposed in the barriers to detect strains wherever they might occur.

[0022] In accordance with another aspect of the invention, one or moreaccess tubes are disposed in or below the barriers with at least one endof the tubes extending from the barriers to allow access thereinto.Sensor devices are then disposed in the access tube or tubes and coupledto the signal processing device through the one end of the tubes. Theaccess tubes provide protection for the sensor device without inhibitingoperation thereof. Also, use of access tubes allows for selectiveremoval and deployment of a variety of sensors.

[0023] In accordance with still another aspect of the invention, thesensor device is adapted to detect radiation that may be leaking or mayhave already leaked through the barriers, and/or the presence of RCRAmetals. Also, a sensor device may be provided to detect volatile organiccompounds using fiber optic spectroscopy deployed in the access tubes.

[0024] In another embodiment of the invention, conveyor apparatus isprovided for removing and carrying away excavated earthen material.Disposed above the conveyor apparatus and above any material beingcarried by the conveyor apparatus is one or more sensor devices fordetecting various conditions and components of the material beingcarried. The sensor device is coupled to a processing device fordeveloping information identifying the condition or components detectedby the sensor device, just as with the sensor device disposed in thecontainment barriers described above.

[0025] In another aspect of the invention, sensor detectable tracerscould be used to verify barrier integrity. Specifically, tracers couldbe placed within the barrier with sensors outside the barrier todetermine whether the tracers have migrated through a breach in thebarrier, or stayed in place.

[0026] In a further aspect of the invention, sensors or sensor arraysare installed in or about a barrier simultaneously with the installationof the barrier. For example the sensors or sensor arrays could bedisposed between layers of a multi-layer barrier as the barrier is beinginstalled in a trench dug for that purpose.

[0027] As indicated earlier, one approach to installing a containmentbarrier around a waste site involves the use of high pressure jetsshooting a slurry into soil surrounding the waste site. This is alsoknown as grouting, and typically involves a grouting beam or arm whichcarries the jets and which is moved along a locus to both remove soiland produce the containment barrier with a mixture of slurry and soil.In accordance with an aspect of the present invention, a sensor orsensors are disposed on the grouting arm to detect physical propertiesof the soil through which the arm moves, to thus determine whethercontaminants have leaked from the waste site into the surrounding soil.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The above and other objects, features and advantages of theinvention will become apparent from a consideration of the subsequentdetailed description presented in connection with the accompanyingdrawings in which:

[0029]FIG. 1 is a perspective view of a plot of ground contaminated byhazardous waste;

[0030]FIG. 2 is a perspective view of the plot of ground with thehazardous waste contained by a protective ground barrier;

[0031]FIG. 3 is a side, schematic view of sensor apparatus positionedabove a conveyor carrying excavated material, in accordance with thepresent invention;

[0032]FIG. 4 is a perspective view of a grid sensor system deployed in acontainment barrier, in accordance with the present invention;

[0033]FIG. 5 is a side, schematic view of a fiber optic strain/cracksensor system deployed in a containment barrier, in accordance with thepresent invention;

[0034]FIG. 6 is a schematic view of a gamma spectroscopy sensor systemsuitable for use in the present invention;

[0035]FIG. 7 is a side view of a barrier placement machine suitable forconstructing a multilayer underground barrier and for simultaneouslydeploying sensor devices in the barrier; and

[0036]FIG. 8 is a side, cross-sectional view, enlarged, of themulti-layer underground barrier of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

[0037] Referring now to FIG. 1, a typical waste site 11 is showncontaining drums 13 filled with hazardous waste, both on the surface 15and buried under the ground 17. Contaminants 19, leaking from the drums13, threaten to migrate into a water table 12, unless some type ofcontainment barrier can be provided.

[0038] One such containment barrier 21 is shown in FIG. 2 to includeside barriers or walls 23 and a floor or horizontal barrier 29. The sidebarriers 23 may be made using conventional methods and interconnected tothe horizontal barrier 29. Additionally, the waste site 11 could becompletely encapsulated by forming an upper barrier cover (not shown)and interconnecting it with the side barriers 23 and front and rearbarriers 25 and 27 (front barriers 25 are shown in phantom line in FIG.2). The afore-cited co-pending patent application describes howcontainment barriers of the type described may be constructed usingapparatus such as that to next be briefly described.

[0039]FIG. 3 is a side, schematic view of one embodiment of excavatedsoil sensor and assay equipment, in accordance with the presentinvention. FIG. 3 shows a conveyor 710 on which excavated soil 700 (froma waste containment site) is being carried for ultimate deposit.Disposed above the conveyor 991 for detecting various physicalparameters and contaminants of the soil 700 are a gamma ray spectrometer704, an X-ray fluorescence detector 708, and a hood 712 for collectingvapors rising from the soil 700 and passing the vapors to an analyzer716. Disposed under (or could be over) the upper section of the conveyor991 is a scintillating fiber bundle 720 coupled to anoptical-to-electrical convertor 728. The gamma ray spectrometer 704,X-ray fluorescence detector 708, analyzer 716 and optical-to-electricalconverter 728 are all coupled to a monitor 732 for processing signalsreceived from the various components shown for displaying informationrepresented by the received signals or for taking other action.

[0040] The gamma ray spectrometer 704 is provided for makingmeasurements of the energies of particles emitted by differentradioactive sources in the soil 700 to thereby distinguish among thesources and identify them. The gamma ray spectrometer 704 suppliessignals to the monitor 732 identifying the different sources ofradioactivity, and the monitor processes these signals to provide adisplay, hard copy printout, or other indication to a user of whatsources of radioactivity are present in the soil 700. Gamma rayspectrometers are well known in the art.

[0041] The X-ray fluorescence detector 708 is provided for detecting thepresence of RCRA metals in the soil 700. The detector 708 suppliessignals to the monitor 732 indicating whether or not RCRA metals havebeen detected, and the monitor then develops a suitable display,printout, etc. This type of detection is well known.

[0042] The hood 712 collects whatever vapors may be emitted by the soil700, but in particular volatile organic compounds, and these aresupplied to the analyzer 716. The analyzer 716 could include a varietyof devices for detecting the presence of volatile organic compoundsincluding an acousto-optic tunable filter (AOTF) infrared spectrometeror a Fourier-transform infrared spectrometer. Either of these devices issuitable for detecting the presence of volatile organic compounds andboth are well known in the prior art. If volatile organic compounds aredetected by the analyzer 716, the analyzer supplies signals to themonitor 732 identifying the volatile organic compounds and thisinformation may then be displayed, provided on a hard copy printout,etc.

[0043] The scintillating fiber bundle 720 is provided to detect thepresence of radiation emanating from the soil 700 being conveyed on theconveyor 991. The fiber bundle 720, in the presence of different typesof radiation, emits light of a characteristic frequency, and this lightis then supplied to the optical-to-electrical converter 728. There, thelight is converted to electrical signals for supply to the monitor 732,for producing a display or other indication of the nature of theradiation detected.

[0044] Scintillating fiber bundles illustratively may be made ofpolystyrene fibers, doped with fluorescent compounds that scintillate inresponse to various kinds of ionizing radiation. This radiation-inducedscintillation comprises the light supplied to the optical-to-electricalconverter 728 for conversion to electrical signals. Scintillating fiberbundles are commercially available.

[0045] The monitor 732 might, advantageously, be a conventionalcomputer-based data acquisition and display system, such as a Dell PCwith Pentium processor.

[0046] The sensing and assaying discussed above is for soil excavated asa result of installing a waste containment barrier, for example inaccordance with the method described in the afore-cited co-pendingpatent application. It is also desirable to monitor the barrier itselffor integrity and to determine whether leakage of contaminated materialthrough the barrier is taking place. FIG. 4 is a perspective view of agrid sensor system for monitoring the integrity of a waste containmentbarrier 800. In one embodiment, the grid sensor system includes a firstplurality of conductors 804 extending generally in parallel in onedirection through the barrier 800, and a second plurality of conductors808 extending also generally in parallel in another direction in thebarrier to intersect with the first plurality of conductors at an endwall 800 a and a bottom wall 800 b (and the other end wall not shown) ofthe barrier 800. Both ends of the first plurality of conductors 804 andof the second plurality of conductors 808 are gathered and routed to asignal source and processor 812. The signal source and processor 812supplies electrical signals to both sets of conductors 804 and 808,which have a predetermined characteristic impedance. The electricalsignals supplied to one end of the sets of conductors will then bereceived by the signal source and processor 812 from the other end. Anystrain, i.e., change in dimension, which takes place in the material ofthe barrier 800, for example, such as the development of cracks oropenings, will affect the conductors 804 and 808. The affect will begenerally to elongate the conductors where the strain occurs and thiswill result in a change in the characteristic impedance of the affectedconductors. If a strain, for example, occurs near an intersection of oneof the conductors 804 and one of the conductors 808, then thecharacteristic impedance of those two conductors could be read by thesignal source and processor 812 and that would locate the location ofthe strain as being near the intersection. The change in characteristicimpedance can be measured with electrical time domain reflectometry, awell-known measuring technique. Once the location or locations of strainare detected by the signal source and processor 812 (e.g., spectrumanalyzer), it signals a monitor 816 which develops an output identifyingthe location of the strain. The monitor 816 might advantageously be acomputer-based data acquisition system, as with the monitor 732 in FIG.3.

[0047] An alternative embodiment to the conductor grid described abovefor determining integrity of the barrier 800, is a grid of fiber opticstrands disposed in the barrier 800 in the same manner as are theconductors. Assume that the conductors 804 and 808 are simply replacedwith fiber optic strands (as shown in a side view in FIG. 5) and thatthe signal source and processor 812 provides light of a certainintensity and wavelength to one end of strands 804 and 808 and then thatthe signal source and processor receives from the corresponding oppositeends the light that has been transmitted through the strands. If achange in wavelength and/or intensity of the light in any of the strandsis detected by the signal source and processor 812, such changeindicates that strain or cracking has occurred in the barrier 800 at alocation near the affected strands. Thus, detecting a change in thewavelength and/or intensity of light in two or more intersecting strandswould indicate that the strain or cracking has occurred near thatintersection and this information could be supplied by the signal sourceand processor 812 to the monitor 816 for display or other disposition.For processing the received light, the signal source and processor 812might illustratively be a commercially available optic time domainreflectometer, or optical spectrum analyzer, interfaced to a personalcomputer.

[0048] The spacing between conductors 804 and 808 or between fiber opticstrands 804 or 808 could illustratively be about one foot. This wouldenable identification of the location of strain or cracks in the barrier100 to resolution of about six inches.

[0049] Although a grid of either conductors or fiber optic strands wereshown and described for FIG. 4, it is also possible to detect thelocation of a strain or crack occurring in a barrier by an array ofwires or fiber optic strands extending parallel to one another and justin one direction. In particular, a strain or crack which affects asingle wire can be located using electrical time domain reflectometry inwhich a wavelength shift in a signal applied to the wire indicates astrain or cracking in the barrier, as analyzed by a spectrum analyzer.Electrical time domain reflectometry is a well-known operation.Similarly, the location of a crack or strain affecting a fiber opticstrand could be determined by measuring a back-reflected signal(reflected from the crack or strain in the fiber) of an optical pulsesent down the fiber, using optical time domain reflectometry.

[0050]FIG. 5 shows another embodiment of a fiber optic strain/cracksensor system embedded in a containment barrier made of grout.

[0051]FIG. 6 shows a side schematic view of another embodiment of thepresent invention in which hollow access tubes 604 are disposed in acontainment barrier 600, with the tubes being placed into the barrier(or below) during emplacement of the barrier. The access tubes 604 areused to deploy, among others, radiation sensors, such as scintillatingfiber bundles or thermoluminescent dosimeters, X-ray fluorescencesensors for detecting the presence of RCRA metals, and/or a fiber-opticspectroscopy system to detect volatile organic compounds. The accesstubes 904 could be emplaced in the barrier 900 using a variety of knowndeployment methods. The access tubes 904 may be placed in the bottomwall of the barrier and/or the sidewalls thereof.

[0052]FIG. 6 shows a specific embodiment of a sensor system carried inthe access tube 604 to include scintillating fiber bundles 608 (bestseen in the enlarged view 612 of a section of the barrier 600 and tube604). The scintillating fiber bundles 608 were discussed earlier inconnection with FIG. 3, and operate to emit light of differentfrequencies depending upon the type of radiation to which the fiberbundles are exposed. Fiber optic strands 616 are carried by the accesstube 604 and coupled to the scintillating fiber bundles so that lightemitted by the fiber bundles when exposed to radiation is carried by thefiber optic strands to a monitor 620. The monitor 620 would include anoptical-to-electrical convertor for converting the light to electricalsignals for processing by a signal processing circuit to developinformation identifying the type of radiation detected which informationcould then be provided to a user.

[0053] X-ray fluorescence sensors could also be deployed in the accesstube 604, for detecting the migration of RCRA metals through the barrier600, in a manner similar to that discussed in connection with FIG. 3.Conductors would be coupled to the X-ray fluorescence sensors forcarrying signals to the monitor 620 for processing and display ofinformation relating to the presence of RCRA metals.

[0054] Fiber-coupled optical systems based upon Raman and/orfluorescence spectroscopies could also be deployed in access tubes in oraround the barrier to detect and identify volatiles permeating throughthe containment barrier and through perforations 628 in the tube 604.Such systems operate by transmitting an excitation signal from a laserto a sample volume at the distal end of an optical fiber, or fiberbundle, and then sampling and analyzing the excited gas in the volumewith a second fiber. This signal is then returned to a spectrometer andanalyzed to determine the type and concentration of volatiles present.The systems can be multiplexed to obtain samples from multiple locationsbeneath the barrier. Using available microchip laser technology, thelaser itself can be fiber-optically coupled and placed in the accesstubes. A fiber optic spectroscopy sensor 624 at the distal end of afiber 626 is shown in the enlarged views 612 and 614 of the access tube604.

[0055] Two other types of sensor systems could utilize the tube 604 ofFIG. 6 including acoustic sensors and radar sensor systems. Acousticsensors could be used to determine barrier emplacement performance andto gather information about waste pit contents. Typically, arrays ofacoustic transmitters would be disposed in tubes extending through thebottom wall containment barrier, for transmitting acoustic signalsupwardly through the waste pit contents. Arrays of acoustic receiversare deployed on the surface or just under the surface at the top of thewaste pit for receiving transmitted acoustic signals. The acousticreceivers in effect measure the propagation of various seismic waves,such as pressure waves, shear waves, raleigh waves, etc. (through thewaste pit contents), such propagation depending upon the elasticproperties of the contents. The arrays of transmitters and arrays ofreceivers are coupled via control cables to signal source and processorequipment and monitors for processing the acoustic signals anddisplaying information determined from the sensors, in a manner similarto the systems discussed earlier.

[0056] A radar system could also be used to map barrier performance.With such a system, transmitters could be deployed in the tubesextending in the bottom wall of a barrier containment system to transmitelectromagnetic waves upwardly through the waste pit contents toelectromagnetic wave receivers deployed on or near the surface of thewaste pit. Heterogeneities in the waste pit contents (e.g., differentsoils, objects, moisture content, etc.) have different electromagneticproperties, transmitting electromagnetic waves through the wastecontents and then receiving and mapping the transmitted signals willprovide data about the contents and the performance of the wastecontainment barrier in containing the contents. Of course, thetransmitter arrays and receiver arrays would be coupled by cables (ortelemetry devices) to signal source and processor equipment and monitorsfor displaying the data derived from the transmission and reception ofelectromagnetic waves through the waste pit contents.

[0057] Although the acoustic sensor system and radar system describedabove were defined as transmitting signals from the bottom of the wastepit up to the surface thereof, it is obvious that the transmitters couldbe arranged on one side of the waste pit, with receivers arranged on theopposite side and that the signals could be transmitted effectivelyhorizontally through the waste pit contents. In this case, thetransmitters would be deployed in access tubes located on one side ofthe waste pit, with receivers deployed in access tubes located on theother side of the waste pit.

[0058] A resistivity system might also be deployed in the tubes formeasuring long-term barrier performance. Such a system utilizes very lowfrequency electromagnetic fields (approaching the direct-current limit)to perform direct current resistivity measurements of the barriercontents. An electromagnetic wave transmitter would be deployed in atube near the center of the waste pit at the bottom thereof, to transmit360 degrees outwardly, with receivers being located outside of the wastepit, either underground or on the surface for receiving the transmittedwaves. The resistivity measurements would provide an indication ofbarrier integrity such as imperfections, cracks and breaks.

[0059] With the arrangement of access tubes described in particular withrespect to FIG. 6, it is apparent that various sensors could be deployedin the access tubes simultaneously or one type sensor might be deployedfor data gathering at one point in time, then removed and another typesensor deployed in the access tubes for acquisition of different data.Since one or both ends of the access tubes would extend through thesurface of the ground, sensor arrays could easily be installed and laterremoved from the access tubes to make way for a different sensor array.

[0060] Advantageously, the access tubes 604 could be made of anyflexible, electrically neutral material, and may be perforated, as shownat 628 in FIG. 6, to allow entry of VOC's for detection purposes. Theaccess tubes 604 could illustratively have an inside diameter of from0.5 to 6 inches. The spacing of the access tubes, advantageously, isabout three feet.

[0061] Another approach to monitoring barrier integrity involves the useof a tracer system in which tracers are placed at various locations inthe barrier. The tracers could be dye, detectable by fluorescencespectroscopy, visual or chemical testing of samples of soil orgroundwater, or ferromagnetic material, detectable by magnetic sensors.The sensors would be placed outside the barrier in positions to detectmovement of the tracers and thus a possible breach in the integrity ofthe barrier.

[0062] Referring now to FIG. 7, there is shown an embodiment of abarrier placement machine 220. The barrier placement machine 220includes an operator's cab 97, a cutting chain and grout injectorassembly 333 including cutter teeth 31 and discharge paddles 33, a groutreceiving conveyor 959, a soil retaining shield traveling pan 953, asoil retaining shield consolidator 955, a side trench excavator 91, soilconveyor 933, and track mechanism 975 for moving the entire machine 220.The machine 220 is depicted in FIG. 7 in schematic form, and may includeall other components necessary for its operation, as understood by thoseof ordinary skill in the relevant field.

[0063] As the barrier placement machine 220 moves forward, a trenchexcavator 91 digs a side trench shown in phantom line at 226. The trenchexcavator 91 carries the excavated soil 984 up out of the ground anddumps it on the trench excavator conveyor 991, which carries the soilbackwardly along the machine 220. Grout or other suitable barrierforming material is then placed within the side trench 226 by the soilretaining shield traveling pan 953 and the soil retaining shieldconsolidator 955, along with any other necessary grout injecting devicesknown to those of ordinary skill, to form the side barrier. The trenchexcavator conveyor 991 dumps the soil 984 behind the barrier placementmachine 220, refilling the side trench 226. Simultaneously, the cuttingchain and grout injector assembly 333 and soil conveyor 933 operate toexcavate earthen material 985 from beneath the in-situ portion of earth216 without removing said in-situ portion, and discharges the soil 985above ground as shown in FIG. 7 where it lies conveniently accessiblefor testing if desired.

[0064] The machine 220 further includes a barrier-forming means 953,955and 224 attached to the excavating means 31, 33 and 91 forsimultaneously forming a side barrier and a generally horizontal,multi-layer barrier 228 (or could be a single-layer) within thegenerally horizontal trench 222, said multi-layer barrier 228 having atleast a first layer 202 and a second layer 204. This is furtherdescribed in the afore-cited co-pending application.

[0065] Regarding the horizontal, multi-layer barrier 228, a horizontalbarrier forming mechanism 224 is provided for forming at least a portionof the second layer 204 simultaneously with forming at least a portionof the first layer 202. More specifically, the horizontal barrierforming mechanism 224 includes: a first injector 232 for injecting afirst material for forming the first layer 202 in the horizontal trench222; a mechanism for placing an intermediate shield 234 over thematerial for the first layer 202; a second injector 236 for injecting asecond material for forming the second layer 204 onto the intermediateshield 234; and a frame 238 to which the intermediate shield 234 isattached for removing the intermediate shield 234 from between the firstand second material forming the first and second layers 202 and 204. Theintermediate injectors 232 and 236 and, as an extension of the frame238, is advanced horizontally between the first and second layers 202and 204 as they are formed, as the track mechanism 975 advances themachine 220.

[0066] The first and second injectors 232 and 236 are contained withinfirst and second chambers 240 and 242, respectively. The intermediateshield 234 thus operates as a carrying member coupled to the chambers240 and 242. The third, middle layer 212 begins a dispensable,pre-formed roll 244 of barrier material that resides in a suitably sizedtrench 246. The roll 244 of barrier material includes a first end 248.Any suitable attaching means known to those of ordinary skill in the artmay be used for attaching the first end 248 of the roll 244 of barriermaterial to the intermediate shield 234, such that barrier material iswithdrawn from the dispensable roll 244 as the machine 220 advances. Insuch manner the roll of material 244, which might comprise a highperformance material such as polyethylene or any suitable geo-textilemembrane material, is pulled between the first and second layers 202 and204 as the machine 220 advances. In this embodiment, the barriermaterial of the roll 244 preferably has sufficient strength to be pulledbetween the first and second layers 202 and 204 without substantialtearing.

[0067]FIG. 8 depicts another embodiment of a barrier placement approachin which a dispenser 250 comprises a pre-formed roll of barrier materialrotatably disposed between horizontal digging elements 31, 33 and thechambers 240, 242. The second injector 236 is positioned to inject thesecond layer 204 on top of an intermediate shield 234 a such that saidshield 34 separates the second layer 204 and the pre-formed layer 212 assaid second layer 204 and said pre-formed layer 212 are beingrespectively injected and dispensed. The intermediate shield 34 athereby operates as a retaining plate.

[0068] Various sensors 846 (FIG. 7) and 850 (FIG. 8), of the typesdescribed, may be disposed on the barrier material (geo-textilemembrane) of the rolls 244 and 250, respectively, for sensing barrierintegrity, radiation, etc. In this manner, any desired sensor can bedeployed between the first and second layers 202 and 204 by beingincorporated into the membrane barrier material forming the roll 244 orthe roll 250. In other words, the sensors 846 or 850 can be installed atthe same time as the barrier 228 is installed.

[0069] Referring again to FIG. 8, sensors 35 may be installed in thecutting teeth 31 to detect characteristics of the soil being removedsuch as volatile organic compounds (VOCs), heavy metals and radiation,to determine if contamination has leaked from the waste site. Thesensors 35 might illustratively be comprised of scintillating fiberoptic bundles, x-ray fluorescence sensors, or fiber-coupled opticalsystems, for transmitting signals to a receiver located, for example, onthe surface to indicate the soil characteristics being detected.

[0070] In a manner similar to sensors 35 on the cutting teeth 31,sensors could be mounted on a grouting beam or arm, such as thosedisclosed in the foresighted International Publication Numbers W094/19547 & W0 93/00483 by Halliburton Nus Environmental Corp., fordetecting soil characteristics of soil through which the grouting beamis moved to form the containment barrier.

[0071] It is to be understood that the above-described arrangements areonly illustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention and the appended claims are intendedto cover such modifications and arrangements.

We claim:
 1. In an underground containment barrier excavating and emplacement apparatus having means for excavating earthen material from about a buried waste site, and conveyor means for carrying the excavated material outwardly of the apparatus, the improvement comprising a sensor system for sensing physical properties of the excavated material including: sensing means disposed adjacent the conveyor means for sensing selected physical properties of the material carried by the conveyor means, and for producing signals identifying the sensed physical properties, and signal processor means for processing said signals and for producing human perceivable representations of the physical properties identified by the signals.
 2. A sensor system as in claim 1 wherein said sensing means comprises a gamma ray spectrometer disposed above the conveyor means for detecting radiation emanating from the material on the conveyor means.
 3. A sensor system as in claim 1 wherein said sensing means comprises an X-ray fluorescence detector disposed above the conveyor means for detecting the presence of RCRA metals in the material on the conveyor means.
 4. A sensor system as in claim 1 wherein said sensing means comprises scintillating fiber bundle means disposed below/above the conveyor means for detecting radiation emanating from the material on the conveyor means.
 5. A sensor system as in claim 1 wherein said sensing means comprises an acousto-optic tunable filter disposed above the conveyor means for detecting volatile organic compounds present in the material on the conveyor means.
 6. A sensor system as in claim 1 wherein said sensing means comprises a Fourier-transform infrared spectrometer disposed above the conveyor means for detecting volatile organic compounds present in the material on the conveyor means.
 7. Apparatus for detecting soil conditions at a buried waste containment site comprising: means for excavating soil from about the site, said excavating means including cutting teeth for cutting into the soil, and sensor means disposed on the cutting teeth for measuring certain conditions of the soil into which the cutting teeth cut. 